Women shouldn't drink more than seven alcoholic beverages a week, and never more than three in a single day, say the current U.S. dietary guidelines. That's easy enough to understand, until you realize that defining "one drink" can be almost impossible, thanks to ballooning glasses and ever-bolder concoctions of bar drinks.

It's all part of the wider "supersize me" phenomenon. Over the past few decades, drink pours have swollen to match the so-called "fill lines" of bigger cocktail glasses that have hit many places, says Lesley Townsend Duval, founder and executive director of the Manhattan Cocktail Classic.

Nowhere is the trend more evident than with martinis, says Dale DeGroff, cofounder and president of The Museum of the American Cocktail. In the 1950s, a typical martini glass held around five ounces; the drinks themselves contained a guideline-friendly 1.5 ounces of liquor. Fast-forward to the 1980s and the glasses had grown to hold nearly eight ounces, up to three of those straight alcohol. Today's martini might come in an 11-ounce glass and contain as much as 4.5 ounces of liquor-roughly triple the amount you're advised to swallow in a single day. (Not a martini fan? Consider this: The recommended daily limit of five ounces of wine would look paltry in one of today's big tumblers.)

What's more, many brews are now stronger, regardless of what size glass they're poured into. At 12 ounces (the recommended daily limit), a traditional Bud, for example, has about a 5 percent alcohol content. But popular microbrews often carry a much bigger punch, ranging from 6.5 to a whopping 20 percent.

All this explains why you can't drink like the dudes on Mad Men and expect to make it through lunch. And why so many women have trouble sticking to recommended alcohol limits—or are totally unaware they're wildly exceeding them. Though you can question your drink's contents ("How many ounces of rum are you dumping into that mojito?"), it's best to err on the side of awareness. Use common sense, don't gulp huge drinks out of enormous glasses, and ignore that fill line when you're in control of your own pours.